1. Field of the Invention
The present invention relates to an inkjet printing head and an inkjet printing cartridge applicable to a printing device for carrying out the printing by ejecting liquid such as ink.
2. Description of the Related Art
Printing heads used for an inkjet printing device for carrying out the printing by ejecting ink are classified to several kinds in accordance with methods for ejecting ink. Of them, a printing head of a thermal system applies a driving energy to a heat element to generate the film boiling in ink, resulting in the ejection of ink from an ejection opening due to a pressure caused by the film boiling to complete the printing.
Generally, the printing head includes a plurality of ejection openings for ejecting ink, ink paths communicated with these ejection openings, and heating elements arranged opposite to the ejection openings. Further, the printing head has rows of ejection openings wherein a plurality of ejection openings are arranged to form rows, connected to a common ink chamber via ink supply ports communicated with the ink paths. The common ink chamber is also communicated to an ink supply path provided in a tank case forming an ink storing section so that ink is supplied from the ink storing section sequentially to the ink supply path, the common ink chamber, and the ink paths.
In such a printing head, since the ink is in contact with outer air via the ejection openings, there may be a risk in that the moisture in the ink evaporates to increase the ink viscosity. If the ink viscosity increases in such a manner, there may be a risk in that the ejection of ink becomes unstable. Particularly, first ink ejection after the interruption of ink ejection for a while becomes unsmooth since the ink viscosity increases in the vicinity of the ejection opening due to the evaporation. That is, when an interval between the succeeding ejections becomes long, the moisture in the ink in the vicinity of the ejection opening evaporates to worsen the easiness of ink ejection. If the state of ejection changes in such a manner due to the variation of the ejection interval, there may be a risk in that the printing on a medium is also variable to lower the printing quality.
Also, it has been known that the moisture in the ink evaporates via resin forming an ink storage section to thicken the ink. Since the colorant concentration changes if the moisture in ink evaporates via the ejection opening or the resin forming the ink storage section, there is a risk in that the color density or color tone of the recording printed on a printing medium changes to worsen the quality of the recording. To avoid such the degradation of the recording, various means have been adopted. In Japanese Patent Laid-Open No. H6-126955, a technology is disclosed wherein the evaporation moisture in ink is avoided by providing a valve on an ejection opening side within an ink path. This valve has a pair of electro-conductive elastic bodies covered with an insulating layer and arranged up and down opposite to each other. By applying voltage to these electro-conductive elastic bodies, the elastic bodies are stretched by the static force added thereto.
In Japanese Patent Laid-Open No. 2004-122533, a structure is proposed wherein the ejection energy added to the printing head is variable by means of a drive signal control section in accordance with time intervals for exposing the ejection opening of the printing head. That is, by changing a pulse width of a driving signal for the printing head, viscous ink in the vicinity of the ejection opening caused by the exposure of the ejection opening is ejected.
In Japanese Patent Laid-Open No. 2002-240252, a structure is proposed wherein the inner temperature of the printing head is detected and, in accordance with the detected temperature, the inner temperature thereof is adjusted. By controlling the inner temperature of the inkjet printing head so that the optimum ejection of ink is obtained, the stable ejection is achievable.
The recent inkjet printing head has been quickly improved in performance such as the printing quality, the printing speed or others. To improve the printing quality, it is necessary to use various kinds of ink, miniaturize ink droplets, and arrange the ink ejection openings at a high density in correspondence to the miniaturized ink droplets. Also, in a printing device carrying out the printing operation by the reciprocation scanning of the inkjet printing head on the printing medium, to accelerate the printing speed, it is necessary to either widen a printing area covered with a single scan by increasing the number of ejection openings or use a higher ejection driving frequency so that more ink droplets are ejected.
To miniaturize the ink droplet, it is necessary to reduce the driving energy to be applied to the heat element or minimize an open area of the ejection opening. If the driving energy to be applied to the heat element is reduced, the easiness of the ejection becomes worse after a predetermined period has passed since the amount of ejected ink as well as the ejection speed is lowered. Also, if the open area of the ejection opening is minimized, a flow resistance of the ink to be ejected increases to worsen the easiness of ejection after the passage of the predetermined time interval. Namely, while the easiness of ejection is worsened by the evaporation of moisture in ink after the passage of the predetermined time interval as described before, it is also worsened by the reduction of the driving energy applied to the heat element or the minimization of the open area of the ejection opening.
As a countermeasure thereto, Japanese Patent Laid-Open No. H6-126955 proposes a structure wherein a valve is provided in the ink path to restrict the moisture evaporation from the ejection opening to prevent the easiness of ejection from being worsened after the passage of the predetermined time period. If the driving energy applied to the heating element is reduced, however, it is difficult to restrict the deterioration of the ink ejection caused by the deceleration of the ink-ejection speed after the predetermined time passage unless any means is taken. Also, when the open area of the ejection opening is minimized, it is difficult to restrict the deterioration of the ejection easiness after the predetermined time passage due to the increase of the flow resistance unless any means is adopted as described hereinabove.
As means for recovering a state wherein the ejection easiness becomes worse after the predetermined time has passed when the driving energy to be applied to the heat element is reduced or the open area of the ejection opening is minimized, the preliminary ejection of ink is generally adopted, which does not contribute to the printing and is carried out while avoiding the printing medium. Accordingly, as the ink droplet is smaller, the ejection easiness after the predetermined time has passed becomes worse, which results in the increase in the number of the preliminary ejection as well as an amount of ink not contributed to the printing operation. This causes a risk of increase in cost due to the increase in amount of contained ink, or increase in the running cost due to the reduction of an amount of ink usable by the user for the printing operation.
According to the structure disclosed in Japanese Patent Laid-Open No. H6-126955, since the valve is provided in the respective ejection opening, the cost is extremely high. Whereby, if the number of ejection openings is increased for the purpose of accelerating the printing speed, further rise in the cost is expected.
On the other hand, the control of the driving signal for the inkjet printing head or the control of the internal temperature proposed in Japanese Patent Laid-Open Nos. 2004-122533 and 2002-240252 is effective for restricting the deterioration of the ejection easiness after the predetermined time has passed caused by the reduction of the amount of ejected ink. There is a risk, however, in that, if it is desired to increase kinds of ink or the number of ejection openings, items to be controlled increase, resulting in the complexity of the structure and/or control.